They don't - not the same way that farms or factories need laborers. Some small fraction of your software workers need to be around to handle the running software and hardware in case of failure. In the context of union bargaining power, the difference is important.

If the factory workers don't show up for work, your factory's output immediately drops to 0%. If none of your software engineers show up, most of your company's code will continue to run, some of it in a degraded state, for a while. (How much depends on your sub-industry, and how much you're outsourcing to AWS). And if you can get 5% of your workers to show up, you might be able to handle 90% of the on-call load.